The sd_bus_message_append_array() function appends an array to a
D-Bus message m. A container will be opened, the array contents
appended, and the container closed. The parameter type determines how
the pointer p is interpreted. type must be one of the "trivial"
types "y", "n", "q", "i", "u", "x", "t", "d" (but not "b"), as
defined by the Basic Types[1] section of the D-Bus specification, and
listed in sd_bus_message_append_basic(3). Pointer p must point to an
array of size size bytes containing items of the respective type.
Size size must be a multiple of the size of the type type. As a
special case, p may be NULL, if size is 0. The memory pointed to by p
is copied into the memory area containing the message and stays in
possession of the caller. The caller may hence freely change the data
after this call without affecting the message the array was appended
to.
The sd_bus_message_append_array_memfd() function appends an array of
a trivial type to message m, similar to
sd_bus_message_append_array(). The contents of the memory file
descriptor memfd starting at the specified offset and of the
specified size is used as the contents of the array. The offset and
size must be a multiple of the size of the type type. However, as a
special exception, if the offset is specified as zero and the size
specified as UINT64_MAX the full memory file descriptor contents is
used. The memory file descriptor is sealed by this call if it has not
been sealed yet, and cannot be modified after this call. See
memfd_create(2) for details about memory file descriptors. Appending
arrays with memory file descriptors enables efficient zero-copy data
transfer, as the memory file descriptor may be passed as-is to the
destination, without copying the memory in it to the destination
process. Not all protocol transports support passing memory file
descriptors between participants, in which case this call will
automatically fall back to copying. Also, as memory file descriptor
passing is inefficient for smaller amounts of data, copying might
still be enforced even where memory file descriptor passing is
supported.
The sd_bus_message_append_array_iovec() function appends an array of
a trivial type to the message m, similar to
sd_bus_message_append_array(). Contents of the I/O vector array iov
are used as the contents of the array. The total size of iov payload
(the sum of iov_len fields) must be a multiple of the size of the
type type. The iov argument must point to n I/O vector structures.
Each structure may have the iov_base field set, in which case the
memory pointed to will be copied into the message, or unset (set to
zero), in which case a block of zeros of length iov_len bytes will be
inserted. The memory pointed at by iov may be changed after this
call.
The sd_bus_message_append_array_space() function appends space for an
array of a trivial type to message m. It behaves the same as
sd_bus_message_append_array(), but instead of copying items to the
message, it returns a pointer to the destination area to the caller
in pointer p. The caller should subsequently write the array contents
to this memory. Modifications to the memory pointed to should only
occur until the next operation on the bus message is invoked. Most
importantly, the memory should not be altered anymore when another
field has been added to the message or the message has been sealed.

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systemd 234 SD_BUS_MESSAGE_APPEND_ARRAY(3)